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Article

Adsorption and Immobilization of Cadmium by an Iron-Coated Montmorillonite Composite

by
Bangzheng Ren
1,2,3,
Chengqiang Shu
1,3,4,*,
Zailin Chen
5,
Qiang Xiao
2 and
Yuli He
1,3,4
1
Sichuan Provincial Engineering Laboratory of Monitoring and Control for Soil Erosion in Dry Valleys, China West Normal University, Nanchong 637009, China
2
Institute of Ecology, China West Normal University, Nanchong 637009, China
3
Liangshan Soil Erosion and Ecological Restoration in Dry Valleys Observation and Research Station, Liangshan Prefecture 616753, China
4
School of Geographical Sciences, China West Normal University, Nanchong 637009, China
5
Engineering Center of Yunnan Education Department for Health Geological Survey & Evaluation, Kunming 652501, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(21), 3105; https://doi.org/10.3390/w16213105
Submission received: 8 September 2024 / Revised: 16 October 2024 / Accepted: 22 October 2024 / Published: 30 October 2024
(This article belongs to the Section Wastewater Treatment and Reuse)
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Abstract

:
In this study, an iron-coated montmorillonite composite (FMC) was prepared, and the adsorption and immobilization of cadmium (Cd) was investigated. The composite was coated with spherical amorphous iron (Fe), which can promote the adsorption of Cd. At the fifth minute of adsorption, the rate of Cd adsorption by the FMC reached 97.8%. With temperature, the adsorption of Cd by FMCs first increases and then decreases. High pH can promote Cd adsorption; under the same ionic strength, the adsorption of Cd was greater by montmorillonite (Mont) than that by the FMC at pH < 4, but greater by FMC than that by Mont at pH > 4. High ionic strength had negative effects on Cd(II) adsorption by FMC and Mont, and ionic strength had less of an influence on the FMC than on Mont. Soil microorganisms promoted the dissolution of Fe and the release of Cd in the FMC. High temperature can promote the dissolution of Fe, but its effect on Cd release is not significant. At 32 °C, the Fe dissolution can promote Cd release in the FMC. Both the FMC and Mont reduced the bioavailability and leaching toxicity of Cd, reduced the exchangeable Cd, and increased the Fe-Mn bound and residual Cd. Overall, the FMC was more effective than Mont at immobilizing Cd.

1. Introduction

According to the Agency for Toxic Substance and Disease Registry, cadmium (Cd) is one of the most toxic heavy metals in the environment [1]. Cd can enter and accumulate in the human body via the food chain [2], causing severe damage to the kidneys and lungs or other pathological symptoms [1,3]. According to the World Health Organization, the average background content of Cd in soil is 0.1–0.4 mg kg−1[4]; however, soil Cd levels in some areas are significantly higher than the world average due to natural or human factors [5,6,7,8]. Cd undergoes adsorption reactions after entering the soil, which can affect its migration and bioavailability [9]. Adsorption is also recognised as a promising technique for the effective capture of metal ions from aqueous solutions [10]. Adsorption reactions are influenced by the conditions of solution chemistry, such as pH [11], ionic strength [12,13], and foreign ions [12,14]. Soil components, such as clay minerals [15], iron and manganese oxides [16,17], and organic matter [18], can also affect the adsorption of Cd in soil. Moreover, the adsorption of metal ions is controlled by thermodynamic [19] and kinetic processes [20,21].
Montmorillonite is widely distributed in weathering formations and sediments [22]. It is a 2:1 layer-type smectite clay consisting of tetrahedrally coordinated sheets of silicon ions surrounding a sandwiched octahedrally coordinated sheet of Al ions [23]. Montmorillonite has a large specific surface area, high chemical and mechanical stability, fine cation exchange capacity, and possesses available sorption sites within its interlayer space [12,24]. These structural characteristics make montmorillonite an attractive material for potential adsorption of Cd [25,26]. Adsorption is an endothermic reaction [27] that occurs through the formation of outer-sphere complexes under low-pH conditions and inner-sphere complexes under high-pH conditions [28]. However, the surface of montmorillonite in natural soils is usually coated by smaller amorphous or crystalline inorganic materials, such as Fe oxides and hydroxides, which are released from primary mineral weathering or from the deposition of translocated material [29,30]. Fe coatings are mostly in the form of a gel or gel film [31]. These coatings increase the specific surface area and produce new adsorption sites [32,33]. Recently, pollutant removal using Fe-coated minerals has become a popular area of research. Previous studies have investigated the use of coated bentonite to adsorb Cd and arsenic [34], Fe/Mn oxide-coated kaolinite to adsorb basic dyes [35], coated natural sand to adsorb lead (Pb) [36], coated sepiolite to adsorb Cd [37], and coated gravel to adsorb Cd (II), copper (II), Fe (II), nickel (II), and zinc (Zn) (II) [38]. Therefore, it is feasible to use Fe-coated montmorillonite to remove Cd from sewage and stabilize Cd in soil.
In this study, an Fe-coated montmorillonite composite (FMC) was prepared via hydrothermal synthesis. The three main aims of the research were to examine the (i) effects of iron content, time, temperature, pH, and ionic strength on the adsorption of Cd by the FMC, (ii) effect of soil microorganisms on Fe dissolution and Cd release in FMCs, and (iii) the fractional distribution and leaching characteristics of Cd in soil after application of FMCs.

2. Materials and Methods

2.1. Synthesis and Preparation of Materials

Figure 1 is a schematic illustration of the preparation process of FMC. Briefly, organic matter was removed from natural sodium bentonite using 30% H2O2, and montmorillonite was then extracted by sedimentation, siphoning, and centrifugation for the preparation of the FMC. Then, 25 g montmorillonite and 1875 mL deionized water were added to a blender container and stirred for 90 min. After mixing, 100 g Fe(NO3)3·9H2O was added and the pH was adjusted to >12 by slowly adding 2.5 mol/L NaOH. The solution was stirred for 30 min, covered, and aged at 60 °C in a vacuum drying oven for 70 h. During the aging, the solution was stirred three times. The solution was then cleaned with deionized water to a conductivity of <20 µS cm−1, dried at 40 °C, ground, and passed through a 200 mesh sieve for later use. By adding three different qualities of Fe(NO3)3·9H2O, three different Fe content FMC were prepared according to the above steps, However, it should be pointed out that due to the extremely fine nanoscale particles of Fe in the sample, losses are inevitably incurred during the centrifugal cleaning process in the preparation process. Therefore, after digestion of the prepared FMC, the Fe content was determined using a flame atomic absorption spectrophotometer as the final Fe content of the FMC. The Fe contents of the three FMCs we produced are 189.7, 276.8, and 285.2 g/kg, respectively. The mineralogical characteristics, surface functional group composition, and microscopic morphology were identified using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and field emission–scanning electron microscopy (FE-SEM). The Brunauer–Emmett–Teller (BET) surface area was measured by nitrogen adsorption using a surface area analyzer.

2.2. Experimental Procedures

2.2.1. Adsorption Experiment

All the experiments were performed using the batch technique in polyethylene centrifuge tubes. A Cd-containing solution was prepared with Cd(NO3)2·4H2O. The ionic strength was adjusted using NaNO3, and the pH of the solution was adjusted using HNO3 and NaOH. All the adsorption experiments were performed in a shaking incubator at 180 rpm, after which the solid and liquid phases were separated by centrifugation at 8000 rpm for 15 min. The suspension was passed through a 0.45 μm needle filter, and the Cd concentration was determined using atomic absorption spectroscopy (AAS) (GGX-9, Beijing Haiguang Instrument Co., Ltd., Beijing, China).

2.2.2. Cd Release under Microbial Action

The soil microbial inoculum was prepared by incubating topsoil (0–20 cm) collected from a paddy field at 30 °C for 2 weeks (with a water to soil ratio of 9:1), which was then shaken for 2 h and centrifuged at 700 rpm for 10 min. The Cd-loaded FMCs were prepared according to Section 2.2.1. After adsorption, the samples were centrifuged and cleaned with deionized water four times, dried at 40 °C, ground, and passed through a 200 mesh sieve. Following this procedure, the Cd content of the FMC was 20 mg kg−1. A total of 5 g FMC quartz mixed sample (the quartz to FMC mass ratio was 9:1) was placed into a 100 mL brown soil bottle and sterilized at 121 °C for 30 min. After cooling, 69.5 mL culture solution (5 g L−1 NH4Cl, 50 g L−1 C6H12O6, and 25 mmol L−1 phosphate buffer) and 0.5 mL soil microbial inoculum solution was added. The samples were shaken for 1 h, filled with nitrogen for 2 min to remove headspace air, sealed, and cultured in a biochemical incubator under dark conditions. To study the influence of temperature, the experiment was conducted simultaneously at 12 and 32 °C. A 0.5 mL water sample was taken at a depth of 2.0 cm from the bottom of the soil bottle. Sampling was performed on a purification table and the bottles were shaken as little as possible. After sampling was completed, the nitrogen was charged for 2 min and the samples were returned to the incubator. The water samples were fixed with 2% HNO3 and stored at 4 °C. The concentrations of total iron (TFe) and Cd were measured by inductively coupled plasma mass spectrometry (ICP-MS; PE6000, PerkinElmer, Waltham, MA, USA).

2.2.3. Bioavailability, Toxicity Characteristics, and Fractional Distribution

Cd-containing solution was sprayed onto the soil and stirred continuously. Samples were aged for 4 weeks, air dried, ground, and passed through a 2 mm sieve. Using this procedure, the Cd content of the soil was 23.27 mg/kg. A total of 30 g soil sample was placed in a beaker and the FMC or Mont were added at 1%, 3%, or 5%. Saturated cultivation for 60 days was conducted twice at room temperature. After each saturated cultivation period, the soil samples were naturally air-dried and used for subsequent bioavailability, toxicity leaching, and Cd fraction distribution experiments.

Cd Bioavailability Measurements

The ionic strength of 0.01 M CaCl2 is similar to that of most soil solutions and can be used as an extractant to predict the concentration of heavy metals in soil pore solutions, and it can reflect the bioavailability of heavy metals in soil [39,40]. To assess bioavailability, 5 g soil was mixed with 50 mL 0.01 M CaCl2 solution in a 100 mL centrifuge tube, stirred for 3 h, and centrifuged at 6000 rpm for 20 min. The supernatants were filtered through a 0.45 μm needle filter, and the concentration of Cd was determined using ICP-MS.

Toxicity Leaching Characteristics

The toxicity characteristic leaching procedure (TCLP) is a standard method developed by the United States Environmental Protection Agency to evaluate the leaching of pollutants in soil or solid waste under landfill conditions [41] and can be used for environmental risk assessment of heavy metal-contaminated soil. Briefly, the TCLP extractant was prepared by adding 5.7 mL CH3COOH to 500 mL deionized water, followed by 64.3 mL 1 M NaOH. The final volume was adjusted to 1 L, and the pH was approximately 4.93. A total of 5 g soil was passed through a 2 mm sieve and mixed with 50 mL TCLP extractant in a 100 mL centrifuge tube. The sample was stirred for 20 h and centrifuged at 6000 rpm for 20 min. The supernatant was filtered through a 0.45 μm needle filter, and the Cd concentration was determined using ICP-MS.

Sequential Extraction

Fraction distribution was used to identify the combination of heavy metals in different soil compositions. The fractional distribution of Cd was studied using Tessier sequential extraction [42]. Five fractions were extracted using 1 M MgCl2 (exchangeable), 1 M CH3COONa (carbonate bound), 0.04 M NH2OH·HCl and 20% (v/v) CH3COOH at 96 ± 3 °C (Fe-Mn oxide bound), 0.02 M HNO3 and 30% H2O2 at 85 °C ± 2 °C (organic matter bound), and 3.2 M CH3COONH4 and 20% (v/v) HNO3 with HCl―HNO3―HClO4 digestion (residual). Each Cd fraction was analyzed using ICP-MS.

2.3. Experimental Data Analysis

The adsorption capacity, sorption percentage (%), and distribution coefficient (Kd) values were calculated using Equations (1)–(3), respectively. These equations are as follows:
q e = ( C 0 − C e ) V / m
a d s o r p t i o n ( % ) = C 0 − C e C 0 × 100 %
K d = C 0 − C e C e V m = q e C e
where C0 and Ce are the concentrations of Cd in the initial and equilibrium solutions, respectively (mg L−1); V is the volume of the solution (L); m is the mass of the sample (kg); Kd is the distribution coefficient.
The pseudo-second order rate model was used to describe the kinetics of the adsorption [43], as follows:
t q t = 1 2 k ' q e 2 + 1 q e t
where qe is the equilibrium adsorption capacity (mg kg−1); qt is the adsorption capacity of Cd adsorbed (mg kg−1) at time t (min); k′ is the pseudo-second order rate constant of adsorption (g mg−1 h−1).
The thermodynamic parameters of the adsorption of Cd were calculated using the following equation [19]:
ln   K d = Δ S 0 / R − Δ H 0 / R T
Δ G 0 = Δ H 0 − T Δ S 0
where R (8.3145 J/mol/K) is the ideal gas constant, and T (K) is the Kelvin temperature. ΔH0 and ΔS0 were calculated from the plot of lnKd versus 1/T.
All experimental data are the averages of triplicate measurements. The figures were prepared using Origin 2023 Learning Edition (OriginLab, Northampton, MA, USA), and the relative errors are p < 5%.

3. Results and Discussion

3.1. Physical and Chemical Properties of Materials

3.1.1. X-Ray Diffraction Analysis

The X-ray diffraction (XRD) patterns of Mont and FMC with different Fe contents are shown in Figure 2. The characteristic diffraction peak of the (001) crystal plane of Mont appears at 2θ = 6.596, and the corresponding layer spacing d value is 13.3896 Å. The diffraction peaks at 19.642°, 21.736°, and 61.697° corresponded to the (002), (110), and (061) crystal planes, respectively. When the Fe content of the FMC was 189.7 g/kg, the characteristic diffraction peak of Mont was still observed, but the intensity of the diffraction peak was somewhat weakened compared to that of pure Mont. With the increasing Fe content in the FMC, the characteristic diffraction peak intensity of Mont decreased further, indicating that the surface of Mont was gradually covered by Fe. However, there was no characteristic diffraction peak of crystalline minerals in the diffraction patterns of the FMC with different Fe contents, indicating that the Fe coating on the surface of the FMC was mainly amorphous Fe.

3.1.2. FTIR Spectroscopy

The FTIR spectra of the FMCs with different Fe contents are shown in Figure 3. The infrared spectra of the FMCs with three different Fe contents were generally similar to those of pure Mont, with some changes. In the FMC with 189.7 g/kg Fe, the O-H stretched vibration peak at 3624 cm−1 moved to 3616 cm−1, while it almost disappeared in the absorption peak of the FMC with 276.8 and 285.2 g/kg Fe. With an increase in Fe content in the FMCs, the absorption peaks of the interlamellar water stretching vibration of Mont at 3427 cm−1 shifted to 3423, 3417, and 3416 cm−1, respectively, indicating that Fe ions may enter the interlamellar region of Mont during sample preparation. A new absorption peak appeared at 3184 cm−1 in FMC samples, and the peak was sharper with a higher Fe content. This peak is likely to be the absorption peak of NO3-, because Fe(NO3)3·9H2O was used in the preparation of FMCs.

3.1.3. SEM

The Mont particles had an irregular, flaky shape (Figure 4A). Some of the edges were upturned and petal shaped, the pieces were extremely thin (<50 nm), and the surface of the particles was smooth (Figure 4B). Compared with Mont, the surface of the FMC was rough after loading Fe, and most of the surface was covered by extremely fine spherical particles, which were <20 nm (Figure 4C,D). This size correlates with amorphous Fe, indicating that Fe successfully coated the Mont.

3.1.4. Specific Surface Area

The specific surface areas of Mont, FMC (189.7 g/kg Fe), FMC (276.8g/kg Fe), and FMC (285.2 g/kg Fe) are 68.2334 m2/g, 256.0666 m2/g, 274.1527 m2/g, and 279.3222 m2/g, respectively. It can be seen that the specific surface area of the FMCs is much larger than that of Mont, and that the higher the Fe content in the FMC, the larger the specific surface area.

3.2. Effect of Fe Content in the FMC on Cd Adsorption

Figure 5 shows the concentration of Cd in equilibrium solution (Ce) after adsorption by FMC with different iron contents. Under both C0 conditions, Ce decreased with an increase in Fe content in the FMC. When the C0 was 200 mg L−1, the adsorption rates of Cd by the four samples were all above 98%. When C0 was 500 mg L−1, the adsorption rate of Cd on Mont decreased to 84%, whereas the adsorption rate of Cd on the three FMCs remained above 99%, while the adsorption capacity and Kd increased with an increase in the Fe content in the FMC (Table 1). This indicated that higher Fe content in the FMC was more favorable for Cd adsorption. This may be because the Fe on the surface of the FMC is amorphous, which means that a large number of unsaturated bonds can contribute more adsorption sites [44]. Some researchers have also reported that when clay minerals, such as kaolinite and illite, are coated with Fe oxide, a large number of new adsorption sites can be generated, and the adsorption capacities for Zn, Pb, and Cd increase [45,46].

3.3. Effect of Contact Time on Cd Adsorption

The kinetic curves of Cd adsorption by Mont and FMC are shown in Figure 6A. At the fifth minute, the Cd adsorption rates by Mont and FMC were 95.9% and 97.8%, respectively. After 20 min, the adsorption rates of both remained above 98%, and the adsorption reached equilibrium, which is consistent with other studies that reported fast Cd adsorption (10–20 min) [20,47]. The adsorption rate did not change significantly with increasing time, indicating that Cd was not easily desorbed. The pseudo-second order equation was used to describe the kinetic adsorption process of Cd [43], and the fitting result is shown in Figure 6B. The R2 of both equations was 0.9999, indicating that the pseudo-second order equation described the adsorption process of Cd by the FMC and Mont well.

3.4. Effect of Temperature on Cd Adsorption

Figure 7 shows the Cd adsorption rates of Mont and FMC at different temperatures. The rate of Cd adsorption by Mont increased with increasing temperature. For the FMC, the adsorption rates were 99.53% and 99.59% under the conditions of 288.15 and 303.15 K, respectively, indicating that increasing the temperature was also beneficial to the adsorption of Cd on the FMC. This may be because increasing the temperature increases the diffusion rate of metal ions, making it easier for them to approach the surface of the adsorbent [48], while the energy of the system facilitates the attachment of metals on the adsorbent surface [49]. Increasing the temperature also facilitates the hydrolysis of metal ions, which in turn reduces the electrostatic repulsion force between the adsorbent surface and the adsorbate [50]. However, increasing the temperature too much resulted in a decreased adsorption rate; the Cd adsorption rate of the FMC decreased to 99.34% at 318.15 K. This indicated that the adsorption capacity decreased when the temperature increased over a maximum threshold. This may be because the amorphous Fe on the surface of the FMC has an unstable structure and dissolves at higher temperatures, thereby reducing the number of adsorption sites. Moreover, high temperatures can cause physical damage to the adsorbent, thereby reducing its adsorption capacity [51,52].
As the Table 2 shows, the value of ΔH0 of Mont is positive, which indicates that the process of adsorption of Cd on Mont is endothermic, while the value of ΔH0 of FMC is negative, which indicates the process of adsorption of Cd on Mont is exothermic. The values of ΔG0 of Mont and FMC are negative, which indicates that under the experimental conditions, the adsorption of Cd is a spontaneous process. ΔS0 is positive, which indicates an affinity of adsorbents towards Cd in the solutions, suggesting probable structural changes in the adsorbents [43]. Meanwhile, the increased disorderliness at the solid–liquid interface during uptake is favored when ΔS0 becomes positive [19].

3.5. Effect of pH and Ionic Strength on Cd Adsorption

As shown in Figure 8, four phenomena were observed in the effects of pH and ionic strength on the adsorption of Cd by Mont and FMC. (1) The Cd adsorption rate by Mont and FMC increased with increased pH. Under acidic conditions, the relatively high concentration of H3O+ in the solution competes with Cd for the adsorption site, resulting in a low Cd adsorption rate at low pH. When the pH increases, the competition from H3O+ decreases and more adsorption sites are occupied by Cd2+, which increased the adsorption rate of Cd2+ [18]. (2) Under the same ionic strength condition, the rate of Cd adsorption by Mont was greater than that of the FMC at pH < 4, whereas the adsorption rate by the FMC was greater than that of Mont at pH > 4. This indicated that Mont was more favorable for Cd adsorption than the FMC at low pH. This phenomenon may have occurred because the pHpzc of Mont used in the experiment was 2.7; at pH > 2.7, the surface of Mont generates negative charges through deprotonation and adsorbs Cd. Moreover, Mont has structural negative charges due to its built-in isomorphism, which is not affected by the environmental pH. In comparison, coating the surface of Mont with Fe for the FMC changed the protonation under acidic conditions and masked the structural charge on the surface of Mont. This causes the Cd adsorption capacity of the FMC to be weaker than that of Mont in a strong acidic environment. (3) High ionic strength was not favorable for the adsorption of Cd by Mont and FMC. This is because the increased ionic strength increases the number of ions competing for adsorption sites with Cd [53]. Moreover, the increased ionic strength also increases the concentration of anions in the solution that can form soluble and stable complexes with heavy metal ions, increasing the difficulty of heavy metal adsorption [54]. (4) Ionic strength had a smaller effect on Cd adsorption by the FMC than the effect on Mont. This may be because the coated Fe (Fe-OH) on the FMC adsorbed Cd mainly through inner-sphere complexation [55]. Compared with the background electrolyte, Cd can be adsorbed closer to the mineral surface [56,57,58], so the adsorption by the FMC was less affected by ionic strength than Mont.

3.6. Effect of Competitive Ions on Cd Adsorption

Figure 9 shows the effect of competitive ions on Cd adsorption by Mont and FMC in 0.01 mol L−1 KNO3, NaNO3, Ca(NO3)2, Mg(NO3)2, Na2SO4, NaH2PO4, NaNO3, and NaCl solutions, respectively. It can be seen that the adsorption capacity of Cd under the background of Mg2+ and Ca2+ is smaller than that of Na+. This may be attributed to the fact that the divalent background cations can occupy twice the number of adsorption sites by forming (=SO)2-Mg/Ca [59]. However, Mont has the smallest adsorption capacity for Cd in the background of K+. This may be because K+ can cause the structure of Mont to collapse, thereby significantly reducing the adsorption capacity of Mont for metal ions under K+ background [60]. The order of Ce is Na+ > K+ > Ca2+ > Mg2+ for FMC. This indicates that the divalent ions background is more favorable for the adsorption of Cd by FMC than that of monovalent ions. This may be because the adsorption mechanism of Cd by FMC is different from that of Mont. The order of Ce is PO43− > SO42− > Cl−> NO3− for Mont. This indicates that Mont has the lowest adsorption capacity for Cd under a PO43− background, and the highest adsorption capacity for Cd under a NO3− background. There is no significant difference in the adsorption capacity of Cd by FMC under these four background anions.

3.7. Fe Dissolution and Cd Release of the FMC with Soil Microorganisms

Under the same temperature condition, the TFe of the microorganism inoculated sample was higher than that of the uninoculated sample, especially for the samples at 32 °C (Figure 10). This indicated that microorganisms could promote the dissolution of Fe in FMCs. Studies have shown that microorganisms obtain the energy they need for growth by reducing Fe(Ⅲ) to Fe(Ⅱ), which is significantly affected by temperature [61,62,63]. Regardless of whether the sample was or was not inoculated with microorganisms, the TFe concentration was greater at 32 °C than at 12 °C. This indicated that rising temperature can also promote Fe dissolution in FMCs, but its impact is smaller than that of microorganisms. As shown in Figure 11, at the same temperature, the Cd concentration of the inoculated sample was significantly higher than that of the uninoculated sample. The Cd concentration of the inoculated sample showed a rapid upward trend, with the Cd concentration on the 30th day reaching 0.18 and 0.16 mg/L at 12 and 32 °C, respectively. Therefore, it can be concluded that inoculation with microorganisms is beneficial for the release of Cd from the FMC. The Cd concentration of the uninoculated samples did not change significantly in the first 20 days, and only slightly increased between 20–30 days.
The relationship between TFe and Cd is shown in Figure 12, and the linear fitting results are listed in Table 3. At 12 °C, the R2 values of samples with and without inoculated microorganism were 0.38 and 0.0098, respectively, indicating a poor correlation between TFe and Cd. At 32 °C, the R2 values were 0.8474 and 0.7825, respectively, indicating that TFe and Cd are highly correlated and that the dissolution of Fe at 32 °C can promote the release of Cd.

3.8. The Effect of the FMC on the Bioavailability and Leaching Toxicity of Cd

Compared to the control sample, CaCl2_Cd decreased by 30.9%, 78.1%, and 92.6% for 1%, 3%, and 5% FMC, respectively, and 32.0%, 71.2%, and 87.1% for 1%, 3%, and 5% Mont, respectively (Figure 13A). The effects of the FMC and Mont on the TCLP-extractable Cd in the soil were then evaluated. Compared with the control sample, TCLP_Cd decreased by 12.3%, 29.8%, and 46.8% for 1%, 3%, and 5% FMC, respectively, and by 5.3%, 15.2%, and 15.9% for 1%, 3%, and 5% Mont, respectively (Figure 13B). Both the FMC and Mont reduced the bioavailability and leaching toxicity of Cd in soil, with the FMC having a stronger effect than Mont. Moreover, increasing the addition of the FMC and Mont increased soil pH; with the addition of 1%, 3%, and 5% mass ratios, the soil pH was 6.47, 7.07, and 7.42 for FMC, respectively, and 6.23, 6.95, and 7.27 for Mont (Figure 13C), respectively. The soil pH with both FMCs and Mont was higher than the control soil (5.91). Cd is more prone to precipitation in alkaline environments (hydroxide and carbonate) [64] and increases the surface charge on Fe, Al, and Mn oxides, which is beneficial for Cd adsorption [65,66], resulting in the immobilization of Cd in soil.

3.9. The Effect of the FMC on the Fraction Distribution of Cd in Soil

The effects of the FMC and Mont on Cd fraction distribution in the soil are shown in Figure 14. In the control soil, exchangeable Cd accounted for the largest proportion (66%), followed by Fe-Mn oxide-bound Cd (24%), whereas residual, carbonate, and organic matter-bound Cd accounted for less than 10%. Tessier reported that the proportion of carbonate-bound Cd in the Yamaska and St. Francois areas is relatively high [42], and Zinati reported that when pH > 7, Cd can exist in the form of CdCO3 and Cd-phosphate [67]. However, the proportion of carbonate-bound Cd in our experiment was only 6%, which may be because the soil used in the experiment was acidic (pH = 5.91). When the FMC and Mont contents increased from 1% to 5%, exchangeable Cd decreased from 58.76% to 40.23% for FMC and 61.66% to 50.19% for Mont, respectively. In contrast, the Fe-Mn oxide-bound Cd increased from 30.79% to 50.06% in FMC and from 27.28% to 37.84% in Mont, respectively, and the residual Cd increased from 2.56% to 2.92% in FMC and from 3.01% to 3.66% in Mont, respectively. The exchangeable and carbonate-bound heavy metals are considered to be weakly bound and are easily bioavailable [68,69,70], whereas residual heavy metals are considered an inert fraction and are unavailable to either plants or migration [71,72]. The Fe-Mn-bound fraction has a scavenging effect and may provide a deposition for heavy metals; it is a chemical form that combines strong ionic bonds [73], which allows the release of heavy metals under anaerobic conditions [74]. However, some studies suggest that heavy metals can form M2+ or MOH+ in a reducing environment, which can form stable surface complexes with dissociative hydrated groups (-OH2) or hydroxyl groups (-OH) on the surface of Fe-Mn oxides, thereby being fixed [75]. In this study, the addition of FMC and Mont reduced the content of exchangeable Cd and increased the content of Fe-Mn-bound and residual Cd in the soil. This indicates that FMC and Mont can reduce the mobility and bioavailability of Cd in soil, and that the FMC is better than Mont; however, the effects of FMC and Mont on the carbonate and organic matter-bound Cd content in soil were not obvious. Table 4 lists the effects of several remediation materials on the fraction distribution of Cd in soil. Of course, the remediation effect also depends on the quantities of materials applied, soil properties, cadmium background, incubation time and conditions, and so on.

4. Conclusions

Soil pollution with Cd can have serious impacts on human health. Therefore, an FMC was prepared and investigated for use in the adsorption and immobilization of Cd. The coated Fe of the FMC was conducive to Cd adsorption, with a higher Fe content being more favorable for Cd adsorption. The rate of Cd adsorption on the FMC reached 97.8% within 5 min, indicating it could be used as an emergency response material for Cd pollution. High temperatures, strong acid environments, and soil microorganisms were not conducive to the adsorption of Cd by the FMC. Additionally, the FMC reduced the bioavailability and leaching toxicity of Cd in soil, reduced the exchangeable Cd, and increased the Fe-Mn bound and residual Cd contents. Therefore, FMCs are potential candidates for the remediation of Cd-contaminated soils. In future work, it will be necessary to study the optimal application amount of FMC for Cd contaminated soil remediation, and to investigate the effects of FMC application on soil pH, bioavailability of Cd in soil, crop growth, and so on.

Author Contributions

Conceptualization, B.R.; Methodology, C.S.; Experiment, B.R.; Investigation, Z.C.; Data curation, B.R.; Supervision, Q.X.; Formal analysis, B.R.; Writing—original draft, B.R.; Writing—review and editing, C.S.; Validation, Y.H.; funding acquisition, C.S. and Q.X. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by doctoral research initiation project of China West Normal University (No. 493064) and Natural Science Foundation of Sichuan Province (No. 2022NSFSC1734), National Natural Science Foundation of China (NSFC) (No: 42171045).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 16 03105 g001
Figure 1. Schematic illustration of the preparation process of FMC.
Figure 1. Schematic illustration of the preparation process of FMC.
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Figure 2. X-ray diffraction pattern of Fe-coated montmorillonite composites (FMCs) with different Fe content.
Figure 2. X-ray diffraction pattern of Fe-coated montmorillonite composites (FMCs) with different Fe content.
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Figure 3. Fourier transform infrared spectroscopy pattern of FMCs with different Fe contents.
Figure 3. Fourier transform infrared spectroscopy pattern of FMCs with different Fe contents.
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Water 16 03105 g004aWater 16 03105 g004b
Figure 4. Scanning electron microscopy images of (A,B) montmorillonite (Mont) and (C,D) FMC.
Figure 4. Scanning electron microscopy images of (A,B) montmorillonite (Mont) and (C,D) FMC.
Water 16 03105 g004aWater 16 03105 g004b
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Figure 5. Adsorption of Cd on FMCs with different Fe content. pH 6.0, m/v = 0.5 g/20 mL, I = 0.01 M NaNO3, and time = 24 h.
Figure 5. Adsorption of Cd on FMCs with different Fe content. pH 6.0, m/v = 0.5 g/20 mL, I = 0.01 M NaNO3, and time = 24 h.
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Figure 6. (A) Effect of contact time on Cd adsorption. (B) Pseudo-second order kinetics of Cd adsorption. C0 = 200 mg L−1, pH 6.0, m/v = 0.3 g/20 mL, ionic strength (I) = 0.01 M NaNO3, and temperature (T) = 298.15 K.
Figure 6. (A) Effect of contact time on Cd adsorption. (B) Pseudo-second order kinetics of Cd adsorption. C0 = 200 mg L−1, pH 6.0, m/v = 0.3 g/20 mL, ionic strength (I) = 0.01 M NaNO3, and temperature (T) = 298.15 K.
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Figure 7. Adsorption rate of Cd at different temperatures. C0 = 200 mg L−1, pH 6.0, m/v = 0.5 g/20 mL, I = 0.01 M NaNO3, and time = 24 h.
Figure 7. Adsorption rate of Cd at different temperatures. C0 = 200 mg L−1, pH 6.0, m/v = 0.5 g/20 mL, I = 0.01 M NaNO3, and time = 24 h.
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Figure 8. Effect of pH and ionic strength on Cd adsorption. C0 = 200 mg L−1, m/v = 0.5 g/20 mL, and time = 24 h.
Figure 8. Effect of pH and ionic strength on Cd adsorption. C0 = 200 mg L−1, m/v = 0.5 g/20 mL, and time = 24 h.
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Figure 9. Effect of competitive ions on Cd adsorption. C0 = 200 mg L−1, pH 6.0, m/v = 0.5 g/20 mL, and time = 24 h.
Figure 9. Effect of competitive ions on Cd adsorption. C0 = 200 mg L−1, pH 6.0, m/v = 0.5 g/20 mL, and time = 24 h.
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Figure 10. Total Fe (TFe) concentration under microorganism action.
Figure 10. Total Fe (TFe) concentration under microorganism action.
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Figure 11. Cd concentration under microorganism action.
Figure 11. Cd concentration under microorganism action.
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Figure 12. The relationship between Fe dissolution and Cd release in FMC samples inoculated with soil microorganisms.
Figure 12. The relationship between Fe dissolution and Cd release in FMC samples inoculated with soil microorganisms.
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Figure 13. Effects of the FMC and Mont on the (A) bioavailability and (B) toxic dissolution of Cd and (C) pH in soil.
Figure 13. Effects of the FMC and Mont on the (A) bioavailability and (B) toxic dissolution of Cd and (C) pH in soil.
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Figure 14. Effect of FMC and Mont on fraction distribution of Cd in soil.
Figure 14. Effect of FMC and Mont on fraction distribution of Cd in soil.
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Table 1. The Cd adsorption capacity, adsorption (%), and distribution coefficient (Kd) on FMCs with different Fe content.
Table 1. The Cd adsorption capacity, adsorption (%), and distribution coefficient (Kd) on FMCs with different Fe content.
C0 (mg L−1)Fe Content (g kg−1)Adsorption Parameter
Adsorption (%)Adsorption Capacity (mg kg−1)Kd
200098.07840.61968.0
189.799.07925.14234.6
276.899.77977.614,230.4
285.299.97995.571,261.2
500084.016,809.0210.7
189.799.719,907.812,026.0
276.899.919,952.937,811.1
285.299.919,960.860,304.4
Note(s): Fe content 0 = pure Mont. C0; concentration of Cd in the initial solution.
Table 2. Evaluated thermodynamic parameters for Cd sorption on Mont and FMC.
Table 2. Evaluated thermodynamic parameters for Cd sorption on Mont and FMC.
Adsorbent NameΔH0ΔS0ΔG0(kJ·mol−1)
(kJ·mol−1)(J·mol−1·k−1)288.15303.15318.15
Mont14.45117.02−19.26−21.02−22.77
FMC−8.5846.27−21.91−22.61−23.30
Table 3. Linear correlation parameters between TFe and Cd for samples inoculated with soil microorganisms.
Table 3. Linear correlation parameters between TFe and Cd for samples inoculated with soil microorganisms.
Temperature12 °C32 °C
SampleABR2ABR2
Gu/FMC−4.23740.51120.009840.948−0.36590.7825
Gu/FMC + Mic5.97320.58850.380053.189−0.77830.8474
Table 4. Comparison of FMC and passivation materials used in other studies.
Table 4. Comparison of FMC and passivation materials used in other studies.
ExchangeableCarbonate AssociatedFe-Mn Oxide AssociatedOrganic Matter AssociatedResidualReferences
Ⅰ−36.91%15.50%+35.44%−35.44%/[76]
Ⅱ−31.60%////[77]
Ⅲ−14.10%///+5.10%[78]
Ⅳ−28.97%−23.53%+10.71%/+294.2%[79]
Ⅴ−38.06%///+68.96%[80]
Ⅵ−66.10%−68.30%+66.70%/+38.60%[81]
Ⅶ−39.04%−41.09%+52.05%+47.43+18.56%This work
Note(s): Ⅰ: sulfhydryl grafted palygorskite; Ⅱ: biochar combined with sulfate reducing bacteria; Ⅲ: Phosphate modified magnetite@ferrihydrite; Ⅳ: biochar and sepiolite; Ⅴ: hydrothermal modification of attapulgite; Ⅵ: Hydroxyapatite.
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Ren, B.; Shu, C.; Chen, Z.; Xiao, Q.; He, Y. Adsorption and Immobilization of Cadmium by an Iron-Coated Montmorillonite Composite. Water 2024, 16, 3105. https://doi.org/10.3390/w16213105

AMA Style

Ren B, Shu C, Chen Z, Xiao Q, He Y. Adsorption and Immobilization of Cadmium by an Iron-Coated Montmorillonite Composite. Water. 2024; 16(21):3105. https://doi.org/10.3390/w16213105

Chicago/Turabian Style

Ren, Bangzheng, Chengqiang Shu, Zailin Chen, Qiang Xiao, and Yuli He. 2024. "Adsorption and Immobilization of Cadmium by an Iron-Coated Montmorillonite Composite" Water 16, no. 21: 3105. https://doi.org/10.3390/w16213105

APA Style

Ren, B., Shu, C., Chen, Z., Xiao, Q., & He, Y. (2024). Adsorption and Immobilization of Cadmium by an Iron-Coated Montmorillonite Composite. Water, 16(21), 3105. https://doi.org/10.3390/w16213105

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